The invention relates to an optical scanning device for scanning an information track of an optically scannable information carrier, which scanning device is provided with a radiation source, an optical lens system with an optical axis for focusing a radiation beam supplied, in operation, by the radiation source into a scanning spot on the information carrier, a first actuator for displacing the lens system in a direction parallel to the optical axis, and a first control unit for controlling the first actuator by means of a first control signal, said lens system being provided with a housing, a first lens which is secured in the housing in a fixed position, a second lens which, viewed in a direction parallel to the optical axis, is elastically suspended in the housing, and a second actuator by means of which the second lens can be displaced relatively to the first lens in a direction parallel to the optical axis, while the scanning unit is provided with a second control unit for controlling the second actuator by means of a second control signal.
The invention also relates to an optical player provided with a table which is capable of being rotated about an axis of rotation, an optical scanning device for scanning an information track of an optically scannable information carrier which can be placed on said table, and a displacement device by means of which the scanning device can be displaced relatively to the axis of rotation substantially in a radial direction.
An optical scanning device of the type mentioned in the opening paragraphs, and an optical player of the type mentioned in the opening paragraphs, which is provided with such a scanning device, are disclosed in U.S. Pat. No. 5,712,842. The first lens of the lens system of the known scanning device is a main lens of the lens system and is commonly referred to as objective lens. The second lens of the lens system is an auxiliary lens of the lens system and is commonly referred to as solid-immersion lens, which is relatively small and is suspended, by means of a leaf spring, in the housing of the lens system between the objective lens and the information carrier to be scanned. By using the auxiliary lens, the lens system has a relatively large numerical aperture, so that a relatively small scanning spot is provided on the information carrier to be scanned. As a result, the known scanning device can suitably be used to scan information carriers having relatively small elementary information characteristics, i.e. information carriers having a relatively high information density, such as a high-density CD. By means of the first actuator of the known scanning device, the lens system can be bodily displaced in a direction parallel to the optical axis, so that the scanning spot can be focused on the information carrier. In this case, the first control signal, with which h the first actuator can be controlled by the first control unit, is determined by a focusing error which can be measured by means of a focusing error detector of the known scanning device. By means of the second actuator of the known scanning device, the elastically suspended auxiliary lens can be displaced within the lens system with respect to the main lens in a direction parallel to the optical axis, so that a spherical aberration of the radiation beam in a transparent protective layer applied to the information carrier can be corrected. In this case, the second control signal, with which the second actuator can be controlled by the second control unit, is determined by a spherical aberration in said protective layer which is measurable by means of a further optical detector of the known scanning device.
A drawback of the known optical scanning device and the known optical player resides in that the auxiliary lens and said leaf spring form, within the lens system, a substantially undamped mass spring system. As a result, substantially undamped displacements of the auxiliary lens with respect to the main lens occur if the lens system is displaced by the first actuator for correcting focusing errors at a frequency which is equal, or substantially equal, to a natural frequency of said mass spring system. Such undamped displacements of the auxiliary lens with respect to the main lens lead to inaccurate correction of the focusing errors, and are consequently undesirable.
It is an object of the invention to provide an optical scanning device and an optical player of the types mentioned in the opening paragraphs, wherein said drawback of the known optical scanning device and the known optical player are precluded as much as possible.
To achieve this, an optical scanning device in accordance with the invention is characterized in that, in operation, the first control unit also controls the second actuator by means of a third control signal which is proportional to the first control signal.
To achieve the above-mentioned object, an optical player in accordance with the invention is characterized in that the optical scanning device employed therein is an optical scanning device in accordance with the invention.
The first actuator and the second actuator preferably are so-called force actuators, such as Lorentz force actuators. Such force actuators supply a force which is determined by, and preferably substantially proportional to, a control signal by which the force actuators are controlled. Since the first control unit of the scanning device in accordance with the invention does not only control the first actuator by means of the first control signal, but also the second actuator by means of the third control signal which is proportional to the first control signal, the second actuator exerts a force on the second lens, as a result of the third control signal, which force is proportional to a force which is exerted on the entire lens system by the first actuator. By a suitable ratio between the first control signal and the third control signal, it is achieved that the second lens and the entire lens system are displaced at equal accelerations as a result of, respectively, the third control signal and the first control signal. Due to this, the second lens, which is elastically suspended in the housing, follows the displacements of the housing of the lens system, and it is precluded that the second lens is displaced with respect to the housing and the first lens if the lens system is bodily displaced by means of the first actuator. In this manner, also the development of substantially undamped displacements of the second lens with respect to the first lens is precluded as much as possible during displacements of the lens system by the first actuator at a frequency equal to, or substantially equal to, the natural frequency of the mass spring system formed by the second lens and the suspension thereof. Apart from displacements caused by the third control signal, the second lens must carry out displacements with respect to the first lens, which are determined by said second control signal. To this end, the second actuator is controlled, for example, by a control signal which is a sum of the second control signal and the third control signal.
A particular embodiment of an optical scanning device in accordance with the invention is characterized in that the second lens is a main lens of the lens system, while the first lens is an auxiliary lens of the lens system, which is arranged between the second lens and the information carrier to be scanned. The main lens is, for example, a relatively large objective lens, while the auxiliary lens is, for example, a relatively small solid-immersion lens. By virtue of the fact that the relatively large objective lens is elastically suspended in the housing, the second actuator and the elastic suspension of the second lens are dimensioned such that they can be handled, so that a necessary miniaturization of the second actuator and the elastic suspension is limited as much as possible.
A further embodiment of an optical scanning device in accordance with the invention is characterized in that the first control unit is connected to the second actuator via a low-pass filter. By using said low-pass filter, high-frequency components of the third control signal are filtered out, in particular high-frequency components having a frequency which is high with respect to the natural frequency of the mass-spring system formed by the second lens and the suspension thereof. As a result, it is achieved that the second lens substantially does not follow displacements of the lens system for correcting relatively high-frequency focusing errors. In this manner it is achieved that a mass, which is to be displaced by the first actuator to correct high-frequency focusing errors, is limited mainly to the masses of the housing and the first lens of the lens system, so that the forces to be supplied by the first actuator to correct high-frequency, focusing errors, and hence the necessary power of the first actuator, are limited. Such a limitation is achieved, in particular, if the elastically suspended second lens is the main lens of the lens system. Consequently, in this further embodiment, high-frequency focusing errors are corrected substantially only by means of displacements of the first lens, which has proved to be feasible in practice because high-frequency focusing errors generally only have a relatively small amplitude.
Yet another embodiment of an optical scanning device in accordance with the invention is characterized in that the first control unit is connected to the second actuator via, successively, a high-pass filter and an adder for adding the second control signal and the third control signal. By using said high-pass filter, low-frequency components of the third control signal, in particular direct current components, are filtered out. In practice it has been found that said spherical aberration of the radiation beam can be sufficiently corrected by displacing the second lens once only with respect to the first lens, which displacement is determined by an average thickness of the protective layer of the information carrier to be scanned. Consequently, the second control signal generally is a direct current signal. Since low-frequency components of the third control signal are filtered out by the high-pass filter, it is precluded that a correction of the spherical aberration by means of the second control signal is influenced or disturbed by low-frequency components of the third control signal.